1. Field of the Invention
The invention relates to methods and apparatus for impregnating continuous lengths of fiber and filament structures such as yarns, with polymeric resins.
2. Brief Description of the Prior Art
A wide variety of methods and devices have been proposed for impregnating yarns and like structures composed of textile fibers and/or filaments with polymeric resins; see for example the U.S. Pat. No. 3,413,196 which describes the impregnation of yarns of glass fibers. In general, one of the problems faced by the artisan has been the achievement of a product wherein the multitude of individual filaments or fibers such as glass fibers is completely wetted and encapsulated with the resin impregnant so as to eliminate air pockets or voids between fibers. Unless this is accomplished, fibers will be in physical contact with each other and free to abrade, one upon the other. This of course leads to a reduced life for the structure made up of the filaments or fibers. Additionally, the strength of the reinforced product will be less than it could be if all the reinforcement filaments or fibers are utilized by inclusion in the resin. The problem is especially difficult when the polymeric resin is a relatively viscous thermoplastic resin of a desirably high inherent viscosity.
To circumvent the problems associated with impregnating a continuous length of multifilaments and/or fibers with highly viscous resin compositions, artisans have considered the use of thermosetting resins, i.e.; low viscosity, synthetic polymeric resins which cross-link and gain in viscosity (actually they harden) under the influence of heat. The U.S. Pat. No. 4,285,749 contains a description of such methods together with their advantages and shortcomings.
However, thermosetting resins often lack the resiliency, flexibility and/or softness desired in many applications where their tendency to be hard and rigid is undesired. For this reason, the method of U.S. Pat. No. 4,285,749 may not be suitable in every instance where it is desired to impregnate a continuous length of multifilament and/or fibers with a polymeric resin. Thermoplastic resins may be preferred even if high viscosity thermoplasts are difficult to throughly impregnate the structure of the multifilament and/or fiber length.
It will immediately be recognized that the employment of relatively low viscosity resin impregnants would eliminate many of the difficulties encountered in achieving a complete wetting and impregnation of a multifilament and/or fiber structure. However, the use, for example of a low viscosity, thermoplastic polymeric resin to impregnate a continuous length of multifilaments and/or fibers raises a new set of problems related to the difficulty in maintaining a void-free resin matrix during the initial shaping of the impregnated structure. Low viscosity compositions tend to "flow away" from the multifilaments and/or fibers until sufficiently cooled to be fixed in position.
The method and the apparatus of the present invention is an advance in the art, enabling one to employ relatively low viscosity thermoplastic polymeric resin compositions to impregnate a continuous length of multifilaments and/or fibers, build the viscosity during or immediately prior to shaping the impregnated structure and obtain a product substantially free of voids between adjacent filaments and/or fibers.